The invention relates to surface acoustic wave arrangements having at least two surface acoustic wave structures which are adjacent in the main wave propagation direction and in which the period of the fingers in the first surface acoustic wave structure is different to that of the fingers in the second surface acoustic wave structure, and/or they are shifted in phase with respect to one another.
In addition to the normal propagation losses, broadband losses occur, due to partial conversion of the surface acoustic wave into volume waves, at the junction between two surface acoustic wave structures which are different or are phase-shifted with respect to one another. The conversion losses in this case increase as the metallization height increases. This is described, for example, in an article by Yasuo Ebata, xe2x80x9cSUPPRESSION OF BULK-SCATTERING LOSS IN SAW RESONATOR WITH QUASI-CONSTANT ACOUSTIC REFLECTION PERIODICITYxe2x80x9d in Ultrasonics Symposium 1988, pp. 91-96.
This situation occurs, in particular, when
the two lattice elements (surface acoustic wave structures) differ in terms of period length, metallization ratio and/or layer thickness, or
the distance between the two surface acoustic wave structures is chosen such that the two lattice elements are phase-shifted with respect to one another.
With many filter techniques, such discrepancies from perfect periodicity are essential for the method of operation of the filter (for example: DMS filter). It has thus been proposed, in DE 42 12 517, that the junction between the two surface acoustic wave structures be designed to be quasi-periodic. However, this technique has been found to be sufficiently successful only if the relative difference between the speeds of the surface acoustic wave and the interfering volume wave is considerably greater than the relative useful bandwidth of the filter, as is the case, for example, with narrowband IF filters on quartz. This is the only situation where the interference of the parasitic volume wave with the transfer function is outside the pass band of the filter and thus does not interfere with the filter response.
However, low-loss filters having a broader bandwidth are required for telecommunications networks based on the EGSM Standard or for PCS/PCN.
DMS filters (double mode surface acoustic wave filters) are frequently used as low-loss, broadband filters with high selectivity for RF purposes, for example on a 42xc2x0 rot YXxe2x80x94LiTaO3 substrate or on a 36xc2x0 rot YXxe2x80x94LiTaO3.
One example of a simple single-track DMS filter is illustrated schematically in FIG. 1. This filter in this case comprises a track having input transducers E1 and E2, which are arranged between two reflectors R1 and R2, and the output transducer A. The connections for the input and output transducers can also be interchanged, with A then representing the input transducer, and E1, E2 the output transducers. It is also possible to connect the output transducer, or else the output transducers, of this one track to the input transducer, or input transducers, of a second, parallel track. This allows the selectivity of the filter to be increased.
DMS filters have two separate resonant frequencies within one acoustic track, which define a transmission band. The left-hand edge of the transmission band is governed by the lattice period, while the right-hand edge comes about due to the resonance between two mutually shifted surface acoustic wave structures (input and output transducers). In comparison to a periodic lattice, these two structures have been shifted through a xcex94x of approximately xcex/4 with respect to one another. The distance xcex94x in this case relates to the finger centers of adjacent (electrode) fingers of the surface acoustic wave structures. In practice, one of the two end fingers is preferably broadened by approximately xcex/4, as is shown in FIG. 1 for the output transducer A at the junction to the two input transducers E1 and E2. This is done in order to fill the gap between the two structures with a metallized surface, since any surface leakage wave is carried better under a metallized surface.
This results in a structure having a greatly broadened finger, which has a considerably greater local lattice period p (defined by the distance between the center points of the two free surfaces to the left and right of the finger) than the other fingers. This represents a considerable disturbance with the periodic lattice. FIG. 2 shows, schematically, the profile of the finger period p in the region of the junction between two such transducer structures (surface acoustic wave structures) plotted against the position coordinate x, the propagation direction of the surface acoustic waves.
In mobile radio systems (for example GSM, nominal bandwidth 25 MHz) which have been used until now, it has admittedly been possible to identify the acoustic losses in the form of volume wave emission at the structure junctions, but this has not been so severely pronounced for the provision of low-loss filters to be possible. However, broader bandwidths will be used in future mobile radio systems, in order to provide more channels (for example EGSM, nominal bandwidth 35 MHz).
Normally, the bandwidth of surface acoustic wave filters is increased by increasing the metallization layer thicknesses and reducing the number of fingers. Both measures increase the losses at the junctions between the structures. In practice, these losses result in a reduction in the Q-factor of the transducer/transducer resonance, which defines the right-hand band edge, and thus in a reduction in the upper pass band range.
Particularly in the case of EGSM filters, the influence of the losses is so great that the reduction in the upper pass band must be compensated for by means of additional, external matching elements. The external matching can admittedly reduce the amount of ripple in the pass band, but a significant remaining disadvantage is the increased insertion loss of such filters resulting from the losses at the junctions. The specification required for EGSM, for example, can also only partially be satisfied. External matching networks are, furthermore, always associated with additional costs, weight, surface area on the circuit and production complexity, and are thus undesirable for most users.
The object of the present invention is to provide low-loss broadband filters which avoid the above-mentioned disadvantages.
According to the invention, this object is achieved by a surface acoustic wave arrangement comprising a piezoelectric substrate; at least two surface acoustic wave structures, which are fitted on the substrate, are arranged one behind the other in the propagation direction of the surface acoustic waves, comprise metallic fingers and have a first and second finger period; the two surface acoustic wave structures having a different phase and/or different finger period; fingers at the ends of the two surface acoustic wave structures forming a junction region from a first to a second surface acoustic wave structure, and the local finger period of the first surface acoustic wave structure initially decreasing continuously in the junction region and finally rises continuously again until the finger period of the second surface acoustic wave structure is reached.
The junction region is formed by 5 to 8 fingers at the ends of the two surface acoustic wave structures. The surface acoustic wave structures can be two interdigital transducers, or a reflector in combination with an interdigital transducer, or two reflectors. Preferably, the widths of the fingers of the two structure initially decrease and increase in the junction region and the structure having metallization ratio xcex7 of 0.7 to 0.8.
The arrangement may be a dual mode surface acoustic wave filter (DMS filter), with interdigital transducers which are used as input and output transducers being arranged between two reflectors in one acoustic track, and the surface acoustic wave structures being selected from interdigital transducers and reflectors. The reflectors are connected to the ground. The metallization height of the surface acoustic wave structures is in the region from 9 to 11% of the wavelength, which is associated with the surface acoustic wave structures, of the surface acoustic waves.
The arrangement can have three interdigital transducers which are arranged one behind the other between two reflectors with the central interdigital transducer, which is connected to a first connection, having a total of 27 to 35 electrode fingers and, in contrast, the two outer interdigital transducers, which are connected to a second connection, have a total of 20 to 24 electrode fingers. The distances between the central interdigital transducer and the two outer interdigital transducers are of different magnitude.
The arrangement can be in the form of a two-track arrangement, with the finger periods of the reflectors in the two tracks being of different magnitude. The arrangement can be in the form of a reactance filter with single-port resonators, with a junction between the different finger periods of an interdigital transducer and a reflector in at least one single-port resonator.
The invention results in a reduction in the transmission losses of the filter, which is evident in the improved insertion loss and, in particular, in the upper half of the pass band. The useful bandwidth is thus increased, and there is no need for any external matching networks.
When designing broadband, low-loss surface acoustic wave filters (for example RF filters for EGSM or PCS/PCN on 42xc2x0 rot YXxe2x80x94LiTaO3), the inventors found that additional losses, in the form of conversion to volume waves, also occur in a quasi-periodic lattice if the local period at the junction is greater than in the two structures on both sides of the junction. In contrast to a hard transition, with an abruptly increased finger period, this conversion does not, however, occur below a characteristic onset frequency, which is inversely proportional to the local period of the lattice.
The losses at the junctions between surface acoustic wave structures with a different phase and/or a different finger period are thus considerably reduced, or entirely avoided, in that, according to the invention, a finger period which is less than the finger period of the adjacent surface acoustic wave structures is used at the junction points. This results in the onset frequency for volume wave emission being above the desired transmission band of the filter. The junction between the surface acoustic wave structures is quasi-periodic, that is to say a quasi-periodic structure is formed in the region of the wave junction between two surface acoustic wave structures, which quasi-periodic structure forms a continuous transition between the finger period p and/or the phase of the first surface acoustic wave structure in that of the second surface acoustic wave structure.
Depending on the extent of the discontinuity at the junction, a quasi-periodic junction region is formed having a sufficiently large number of fingers. A total of 3-4 fingers at the ends of each surface acoustic wave structure has been found to be sufficient to suppress the losses even for very severe discontinuities. The junction region should not necessarily be chosen to be greater than is required to avoid losses since, otherwise, this will have a negative effect on the transmission response of the filter.
The desired junction according to the invention is achieved if the finger period of the first surface acoustic wave structure initially decreases continuously in the junction region and finally rises continuously again until the finger period of the second surface acoustic wave structure is reached.
For the purposes of the invention, the term surface acoustic wave structure covers both interdigital transducers and reflectors. Junctions may therefore occur, and be designed according to the invention, not only between two interdigital transducers, but also between an interdigital transducer and a reflector, and between two reflectors. The invention can thus advantageously be used, in particular, for DMS filters and single-port resonators. In the latter, the reflectors have a different (greater) finger period than the interdigital transducer.
The finger width and the finger spacing are reduced continuously in order to reduce the finger period in the junction region.
The choice of a metallization ratio xcex7 (xcex7=ratio of the metallized surface to the unmetallized surface area within one period of the surface acoustic wave structure) between 0.7 and 0.8 reduces the speed of the surface leakage wave, as a result of which the separation between the onset frequency for volume wave conversion and the pass band is further increased. This therefore also reduces the influence of volume wave losses.
Reduced transmission losses are likewise achieved if the reflectors of the surface acoustic wave arrangement are connected to ground, since this results in a considerable reduction in the lossy exchange of charge within the reflector.
The invention will be explained in more detail in the following text, with reference to exemplary embodiments and the drawings.